As is known, the motion transmission in the motorcycle field usually takes place by means of a pinion-chain-crown group (moving from the crankshaft to the rear wheel, respectively), or by means of a cardan-type joint.
The advantage of the first solution consists in the fact that it carries out a transmission instantaneously homokinetic, therefore comfortable, reliable and at a relatively low cost. Moreover, such a transmission is also very light. The drawback essentially consists in that the chain transmission requires continuous maintenance (in terms of cleaning and lubrication) and in that it has a quite limited duration over time due to wear of the chain and/or the teeth of the pinion or crown.
The cardan joint transmission has the advantage of being virtually maintenance free (it being housed in a casing in an oil or grease bath); however, such a transmission is much heavier and more expensive. Moreover, such a transmission is on the average but not instantaneously homokinetic due to the fact that the transmission ratio between the shafts connected by the coupling has a sinusoidal pattern over time: this entails a non-optimum comfort for the user.
In addition, the cardan joint transmission implies a further limitation consisting in the relative position between the centre of the universal cardan joint and the hinging axis of the rear swingarm which must match: the matching in fact allows the oscillation of the swingarm itself. This geometric constraint is strongly limiting since it imposes a squat angle value that considerably determines the dynamic behaviour of the vehicle in terms of load transfer and the work of the rear suspension.
To overcome the latter limitation, it is known from U.S. Pat. No. 7,971,674 to provide a transmission that includes, in series, a cardan joint (kinematically connected to the motor shaft) and a homokinetic joint kinematically connected to the rear wheel. The homokinetic joint allows an axial sliding between the axles connected thereto, always ensuring their integral (and homokinetic) rotation. In this way, it is possible to misalign the centre of the cardan joint with respect to the hinging point of the swingarm: in fact, the rotation of the swingarm is allowed by the axial sliding of the homokinetic joint. In this way, the limitation of the squat angle is overcome and the dynamic behaviour of the vehicle can be modified as desired. On the other hand, however, this solution has the drawback of both the cost/weight due to the presence of the cardan joint and the problem that the transmission is not instantly homokinetic. In fact, the ‘sinusoidal oscillations’ due to the cardan joint are always present and are rigidly transmitted by the homokinetic joint.